Reference electrodes are widely used in many different types of electrochemical experiments including measurements with pH electrodes, ion selective electrodes and cathodic protection instrumentation to access and control the corrosion rate of metallic structures. The role of the reference electrode is to provide a stable potential against which other potentials are measured or controlled. A reference electrode typically consists of at least three components 1) a half cell electrode involving a redox couple, and commonly these are of the metal/metal ion type (e.g. silver/silver chloride or copper/copper sulfate) 2) an electrolyte (e.g. 4 M potassium chloride for the Ag/AgCl or saturated copper sulfate solution for the copper/copper sulfate electrode). 3) a junction that separates the internal solution from the external solution being measured. In, for example, the silver/silver chloride reference electrode, a silver wire is coated with a thin layer of silver chloride and the wire is immersed in a solution of saturated potassium chloride (˜4 M). For the discussion below the silver/silver chloride reference electrode will be used since it is one of the most commonly used reference electrodes, but the discussions apply to other types of metal/metal ion reference electrodes as well.
The potential E of the electrode s governed by the Nernst equation:E=E°+(RT/nF)ln{[ox]/[red]}where E° is the standard potential, R is the gas constant, T is the temperature, n is the number of electrons, F is the Faraday constant, [ox] is the concentration of the oxidized form of the redox couple and [red] is the reduced form of the redox couple. The electrode reactions for the silver/silver chloride reference electrode are:Ag=>Ag++eAg++Cl−=>AgClOn including these equilibria, the Nernst equation becomesE=E°+(RT/nF)ln{[AgCl]/(K[Ag][Cl−]}where K=[AgCl]/(Ag+][Cl−], in which K is the solubility product for silver chloride.
The internal solution must be in ionic contact with the external solution in order for the electrode to function. If the ionic contact is too small, the impedance will be too high and the potential measurement with the reference electrode will be noisy and subject to drift. Therefore a good ionic contact is preferable. However, this ionic contact also allows transport of ions between the internal and external solutions. Contamination of the external solution can be a problem if the measurement is sensitive to, for example, chloride ions leaking from a silver/silver chloride reference electrode. Similarly heavy ions, such as mercurous ions from a calomel reference element, can be a problematic when measuring certain biological media.
Furthermore, unless the external solution has a high chloride concentration (e.g. seawater), there will be a steady loss of chloride from the reference electrode internal solution to the external solution since there is a concentration gradient and diffusion of the external solution into the reference electrode potentially causing contamination of the reference electrode and possible variation in the electrical potential. In order to keep the potential constant, the potassium chloride concentration must be kept constant, which is normally done by using a saturated solution. The internal chloride concentration and hence the electrode potential will be kept constant by the dissolution of any solid potassium chloride present, but once that has all dissolved, the internal chloride concentration will decrease and the potential of the electrode will change accordingly as described by the Nernst equation (vide supra).
Several methods have been employed in the prior art to mitigate this problem. The traditional method used to prevent mixing of the inner solution of the reference electrode with the test solution was to separate the two by a salt bridge. The salt bridge was typically a tube filled with a gelled solution of another salt (e.g. potassium nitrate) that would not interfere with either the test solution or the reference electrode, but the salt solution provided the ionic contact necessary for the function of the reference electrode; however this approach tends to be cumbersome.
Another common way to avoid the problem of loss of chloride is to add a reservoir of saturated potassium chloride and is to make the reference electrode inner chamber refillable. When the internal chloride solution decreases or the electrolyte dries out, the user simply adds more saturated potassium chloride solution. Many commercial reference electrodes are refillable. Thus potassium chloride solution is constantly flowing out of the reference electrode. This configuration works well for some applications, but is unsuitable for situations where the external solution should be kept free of chloride ion, or where the reference electrode is to be left in place for a long period of time without maintenance. For example, reference electrodes for cathodic protection systems to prevent the corrosion of underground steel pipes may only be serviced every six months or more.
Another method that has been extensively utilized is to provide a barrier between the inner solution and the external solution. If the barrier characteristics are chosen correctly, then the ionic contract is sufficient to achieve good performance from the reference electrode, yet the rate of chloride ion transport out of the reference electrode is slow enough that the reference electrode can be used in a wide variety of applications.
In most reference electrodes, the sample electrolytes make contact to the inner solution either by slow flow of salt bridge solution through the barrier into the sample, called a flowing junction electrode, or by mutual diffusion into the porous structure from each side, called a static junction electrode. Typical barriers include ceramic and glass sinters and capillaries, ground glass sleeves, wooden plugs (Cardiero, U.S. Pat. No. 3,440,525) glass—plastics composites (e.g. Neti and Bing, U.S. Pat. No. 4,002,547) and Teflon powder treated to make its surface hydrophilic (Maruyama and Watanabe, U.S. Pat. No. 4,053,382) and more recently microfluidic flowing junctions (Broadley et al, U.S. Pat. Nos. 6,599,409 & 6,616,821) and ion conductive membranes (Connelly and Bower U.S. Pat. No. 6,579,440). Liquid junctions, utilizing ion conductive membranes have also been used (e.g. by Spaziani and Fowler, in U.S. Pat. No. 4,233,136 and Leonard in U.S. Pat. No. 4,913,793). Ion exchange polymers, both water soluble and in-soluble types, have been used as junctions in reference electrodes resulting in good performance in low osmotic pressure solutions Leonard U.S. Pat. No. 4,913,793.
To further reduce the ingress and egress of the external and internal solutions respectively one common approach is to have more than one junction, and so called ‘double junction’ reference electrodes are now widely used. (e.g. Watanabe and Leonard in U.S. Pat. No. 3,103,480, Arrance, in U.S. Pat. No. 4,282,081, Brezinski, U.S. Pat. No. 4,401,548). In addition to loss of electrolyte and contamination another common failure mode is blockage of the junction. Replaceable junctions (e.g. Brezinski, U.S. Pat. No. 4,495,052) are one common method that is employed to overcome clogging of the liquid junction (see below).
Another common method used to reduce the loss of chloride from the reference electrode is to incorporate a gelling agent into the reference electrolyte. The gelling agent reduces the diffusion rate of all species present and so extends the lifetime of the reference electrode. However, as with the other methods that extent the lifetime of the reference electrode by reducing the diffusion rate, a balance is required between the diffusion rate and electrical noise resulting from high impedance if the diffusion rate is too small. There are many examples of gelling agents used including gelatin, agar, sodium carboxymethylcellulose, polyacrylamides, and more recently hydrogels (Thrier et al U.S. Pat. No. 6,468,408). Reference electrodes have also been made with electrolytes solidified with various hydraulically setting cements (Tauber and Dornauf, U.S. Pat. No. 4,927,518).
There are several failure modes of reference electrodes related to the loss of electrolyte by diffusion. The first method is as outlined above, involves direct loss of the electrolyte by diffusion. However, diffusion is not the only mechanism that leads to the loss of electrolyte. If the reference electrode is used in a low ionic strength solution and the internal solution is a high ionic strength solution (e.g. saturated potassium chloride), then the osmotic pressure is going to act to push the low ionic strength external solution into the reference electrode. Since the reference electrode chamber is usually sealed apart from the diffusion barrier (e.g. sinter), the net effect is that the osmotic pressure flushes the electrolyte out of the reference electrode chamber causing a shortened lifetime. Temperature changes can cause pressure imbalances between the inside of the reference electrode chamber and the external environment. These pressure changes can lead to additional mass flow across the barrier.
Another common method by which silver/silver chloride electrodes fail is blockage of the sinter by silver chloride. Silver chloride (AgCl), though insoluble in water, has some solubility in high concentrations of chloride ion through the reversible formation of the AgCl2− complex and higher chloride ion complexes. If the reference electrode is used in a low chloride test solution, then the soluble silver chloride will revert back to the insoluble silver chloride where the internal and external solutions meet, i.e. in the barrier, resulting in blockage of the barrier and failure of the reference electrode. Blockage of the sinter can occur rapidly, for example Brezinski in U.S. Pat. No. 4,401,548 reports that a new silver/silver chloride reference electrode can lose most of its flow capability after less than 24 hours in solution.
Metal ions in the external solution that form insoluble salts with chloride ion (typically the heavy metals: silver, lead, and mercury) can also precipitate in the liquid junction leading to failure of the reference electrode. Of course contamination can simply block the barrier causing the reference electrode to fail even without specific chemical reaction.
Contamination of reference electrodes, especially by compounds that form insoluble species on mixing with the redox active metal salt is another common cause of failure, for example, sulfide containing test solutions can adversely affect silver/silver chloride and calomel reference electrodes through the precipitation of silver sulfide compounds in the barrier.
The primary driving force for many of these failure mechanisms is the high electrolyte concentration required in the reference electrode electrolyte (e.g. 4 M KCl). This high concentration of electrolyte especially limits the use of reference electrodes in low ionic strength solutions (e.g. in fresh water) and for long term applications, such as corrosion monitoring. A method is needed that will allow a reference electrode to be used in low ionic strength solutions, that can be combined with conventional barriers and conventional electrolyte gelling methods and which utilizes conventional redox chemistry but which can extent the lifetime of the reference electrode by employing a low internal ionic strength electrolyte.